Flash tube technology is used for anticollision lights on aircraft. These tubes typically produce sudden, brilliant flashes of light that are much more conspicuous than other types of light sources. Current Federal Aviation Administration (FAA) mandated airworthiness standards require that such flash tubes have an effective intensity of 400 candela when viewed within 5 degrees of horizontal for aircraft certified after 1977. For aircraft certified before 1977 the requirement is 100 candela.
In a new anticollision light, the intensity of the flash tube will meet or exceed FAA brightness standards. Unfortunately, the intensity of the flash tube significantly degrades with use, long prior to actual tube failure. See: B. W. Henderson, "FAA: Aircraft Strobe Lights May Fall Short of Standards", Aviation Week & Space Technology, pp. 42-43, Sep. 14, 1992; and V. M. Gardov, et al., "Theory of Powerful Nonsteady Xenon Discharge Taking Vaporization of Its Stabilizing Walls Into Account", translated from Teplofizika Vysokikh Temperatur, Vol. 19, No. 1., pp. 28-35, January-February, 1981. The result is that most anticollision flash tubes continue in service in a substandard mode long after they have degraded below FAA minimum intensity requirements.
An FAA survey showed that airlines generally rely on the technical manuals from the flash tube manufacturers. Usually, manufacturers do not recommend checking brightness or regularly replacing flash tubes. See Henderson, supra. Because the intensity of a flash tube cannot easily be determined with any certainty with the human eye, airline maintenance personnel cannot easily identify non-complying anticollision lights in need of replacement.
For an anticollision light mounted on the body of an aircraft, it is critical that the light illuminate throughout the horizontal plane covering a 360 degree circumference around the aircraft. To best accomplish this requirement, flash tube manufacturers have universally adopted a flash tube 1 having a donut shaped portion 2 surrounding parabolic-shaped reflector 9. See FIG. 1. Unfortunately, this configuration does not allow a consistent illumination in the horizontal plane throughout 360 degrees. For example, the design of the flash tube anode 3 and cathode 5 forms a small gap 7 in the otherwise circular donut 2. This gap creates a natural "low spot" or "node," resulting in reduced illumination characteristics at this coordinate point. Nodes, similar to those resulting from the tube design, are detected when the intensity of the flash tube is measured circumferentially throughout the horizontal plane.
The node variable intensity phenomena described above is graphically depicted in FIG. 2. As can be seen, the radial light intensity varies throughout a 360 degree arc in the plane of donut 1. FIG. 2 also depicts the intensity units radially, ranging from 0 candela to 600 candela. As an example of the node variable intensity phenomena, FIG. 2 shows node occurrences at zero degrees, 90 degrees (corresponding to the location of gap 7 shown in FIG. 1) and 180 degrees.
The node problem is further intensified when, during normal operation, material within the flash tube erodes due to the high intensity discharge currents inherent in flash tube technology, resulting in flash tube degradation. The eroded material tends to deposit itself upon the glass wall of the flash tube, resulting in several darkened regions on the flash tube. This darkening effect occurs worst in the area around node 3 and cathode 5, an area already low in intensity due to the tube geometry.
Additionally, the particular design of the lens cover surrounding the flash tube may hinder the requirement of consistent illumination circumferentially around the aircraft. When glass lenses of a teardrop or other non-circular design are used for aerodynamic purposes, such as lens cover 15 (FIG. 1), the uniformity of intensity around the flash tube is further eroded by factors such as prismatic effects. Therefore, in normal operation, the lens cover aberration, in conjunction with the flash tube anomaly, may combine to produce a non-uniform light intensity pattern circumferentially around the light. Therefore, some of the node anomalies depicted in FIG. 2 are likely the result of either excess eroded material or lens cover design. At a minimum, node locations (if known) should be tested to conform to current FAA flash tube intensity guidelines. However, it is clearly best to test flash tube intensity 360 degrees circumferentially around the flash tube.
While FIG. 1 depicts a conventional flash tube 2, manufacturers produce various-sized flash tubes for placement on different locations throughout an aircraft's body (e.g., on the aircraft tail, underneath the aircraft fuselage, on top of the aircraft fuselage). As such, these various-sized flash tubes do not emit light which is precisely consistent in intensity. For example, a smaller flash tube used on an aircraft tail would not likely emit the same intensity as a larger flash tube placed underneath the plane.
One proposed solution to testing illumination is to measure a sample of anticollision lights over time to determine the mean time to failure for such lights. This approach is based on statistical probability, and is therefore subject to either replacement of lamps that are operating pursuant to FAA guidelines, or to leaving noncomplying lights on the aircraft. Clearly, this proposed solution provides no certainty of the performance of any individual lamp, may result in unnecessary and expensive lamp replacement, and is subject to probability error.
U.S. Pat. No. 3,366,835 to H. L. Morris discloses a circuit for indicating a flash tube failure when the monitored tube is located where it is not readily visible to the operator. The failure indicator includes a light conducting plastic rod 48 which extends from the vicinity of the flash tube 14 to a photocell 49. Photocell 49 is part of a circuit including relay 53, capacitor 54, contacts 55 and warning light 56. If the flash tube fails to light for a predetermined period, capacitor 54 does not recharge and relay 53 is de-energized. Contact 55 then closes and the indicator light 56 comes on.
Numerous other lamp failure indicators are disclosed in the prior art. See, for instance: U.S. Pat. No. 3,541,504 to R. H. Bush, which is described as a vehicle burn-out indicator; U.S. Pat. No. 3,588,816 to R. H. Himes; U.S. Pat. No. 3,624,629 to C. A. Donaldson; U.S. Pat. No. 4,572,987 to D. M. Embrey, et al.; and U.S. Pat. No. 4,376,910 to J. P. Pieslier.
Copending application Ser. No. 08/173,087 discloses an on-board flash tube monitoring system which includes: a human eye spectral response photodiode for producing analog signals; electronics for converting the analog signals to a digital time function proportional to the intensity of the corresponding flash; and electronics, for monitoring the digital time function and for sending a fault signal when the time function is below a pre-selected minimum. This system is incorporated into the anticollision light unit on-board the fuselage of an aircraft.
Strotek (Carson City, Nev.) claims to have developed an optical measuring system which can check flash tube intensity from outside the aircraft while they are on the ground. See, Henderson, supra.
Prior to the present invention, portable and hand held systems capable of providing an accurate measurement of flash tube intensity from outside the aircraft while the aircraft is at rest on the ground did not exist. The present system is self monitoring and requires minimal maintenance. It is also cost effective to the airline industry, requires no modifications to existing aircraft structures and is capable of monitoring at approved FAA guidelines.
The present invention provides a simple solution to the problem of accurate readings created by the phenomena of light, namely, that light intensity decreases as the square of the distance from the light source increases. Due to this phenomena, a key element of the portable unit of the present invention is its ability to quickly position its photo sensor relative to the flash tube at a precalibrated distance and to maintain such calibrated distance throughout the 360 degree testing arc. Of course, while this system can obtain intensity information from a various-sized flash tubes, it is also capable of obtaining intensity information from any continuous light source. Additionally, this system is, preferably, internally powered, requires minimal set up and can easily be used by airline industry personnel. Since this is a portable system, it is capable of conserving internal power such that repeated replacement of the internal power source is infrequent.
Thus, it is an object of the present invention to provide a portable system for the continuous monitoring of flash tubes, or continuous illumination sources, which will provide a fault signal or other indication when the intensity of the source being monitored falls below a predetermined value. Such a predetermined value may be set by a governmental regulatory agency such as the Federal Aviation Administration (e.g., such as an intensity value falling below 100 candela for planes certified before 1977, or, intensity value falling below 400 candela for plane certified during or after 1977).
It is another object of this invention to provide a portable monitoring system for flash tubes or continuous illumination sources which can accurately monitor fight intensity from a predetermined, fixed coordinate distance without interference from outside sources, such as natural and man-made light sources.
It is an object of this invention to provide a portable monitoring system for flash tubes, or continuous illumination sources, which is operational only when the system is at a fixed, predetermined distance from the light source to acquire, for instance, accurate flash tube intensity information. With the present invention, it is contemplated to utilize a fixed distance indicator which also energizes the monitoring system.
It is also an object of this invention to provide a portable continuous monitoring system for flash tubes, or continuous illumination sources, which is transportable, requires no outside power source, and is capable of monitoring its internal power supply, and notifying an operator of a low battery condition.
This invention improves both aircraft safety and reduces the costly replacement of lights in aircraft systems where, without applicant's monitoring system, lamps are either not replaced when necessary pursuant to governmental regulations, or lamps are unnecessarily replaced even if the lamp is operating within acceptable guidelines.